In recent years, much attention has been drawn towards the research and development of organic light-emitting devices. Such enormous increase in research interest is highly correlated to the potential application of OLEDs in commercial flat panel displays. With the advantages of low cost, light weight, low operating voltage, high brightness, robustness, color tunability, wide viewing angle, ease of fabrication onto flexible substrates as well as low energy consumption, OLEDs are considered as remarkably attractive candidates for flat panel display technologies.
Typically an OLED contains several layers of semiconductor sandwiched between two electrodes. The cathode is composed of a low work function metal alloy deposited by vacuum evaporation, whereas the anode is a transparent conductor such as indium tin oxide (ITO). Upon application of a DC voltage, holes injected by the ITO electrode and electrons injected by the metal electrode recombine to form excitons. Subsequent relaxation of excitons results in the generation of electroluminescence (EL).
In order to achieve higher OLED performance, multiple organic semiconductor layers can be incorporated that further separate the two electrodes. There are two main categories of materials that are used as these semiconductor layers, namely vacuum-deposited small molecules and spin-coated polymeric materials. Both fabrication methods have their respective advantages. Vacuum deposition generally allows better control over layer thickness and uniformity, while spin coating generally offers less complex fabrication having lower production cost [Burrows, P. E.; Forrest, S. R.; Thompson, M. E. Current Opinion in Solid State and Materials Science, 236 (1997)].
In spite of the fact that electroluminescence from organic polymers was initially reported in the 1970s [Kaneto, K.; Yoshino, K.; Koa, K.; Inuishi, Y. Jpn. J. Appl. Phys. 18, 1023 (1974)], it was only after the report on yellow-green electroluminescence from poly(p-phenylenenvinylene) (PPV) that light-emitting polymers and OLEDs received much attention [Burroughs, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, N.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature 347, 539 (1990)]. Subsequently, similar studies reported PPV derivatives as light-emitting polymers [Braun, D.; Heeger, A. J. Appl. Phys. Lett. 58, 1982 (1991)]. Since then a number of new electroluminescent polymers have been investigated for improved properties.
Electroluminescence of organic materials was discovered in anthracene crystals immersed in liquid electrolyte in 1965 [Helfruch, W.; Schneider, W. G. Phys. Rev. Lett. 14, 229 (1965)]. Although lower operating voltages can be achieved by using a thin film of anthracene with solid electrodes, very low efficiency have been encountered for these single-layer devices. High-performance green electroluminescence from an organic small molecule, tris-(8-hydroxyquinoline) aluminum (Alq3), was first reported in 1987 [Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 51, 913 (1987)]. A double-layer OLED with high efficiency and low operating voltage was described where Alq3 was utilized both as emitting layer and electron transporting layer. Subsequent modification of the device to have a triple-layer structure gives better performance with higher efficiency.
Superior performance of phosphorescence-based OLEDs occurs when the semiconducting materials have short lifetimes. Short lifetimes can be achieved by mixing singlet and triplet excited states and exploiting spin-orbit (L-S) coupling. In the presence of a heavy metal center, the propensity of spin-orbit coupling can be greatly enhanced. Hence, the use of heavy metal complexes in OLEDs is generally advantageous relative to the use of purely organic materials. The lowest energy excited state of an organometallic compound is commonly a metal-to-ligand charge transfer (MLCT) triplet state, which can mix with the excited singlet state through L-S coupling, to result in higher photoluminescence efficiencies [Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Pure Appl. Chem. 71, 2095 (1999)]. In 1998, Baldo et al. demonstrated a phosphorescence electroluminescent device with high quantum efficiency by using platinum(II) 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine (PtOEP) as a dye [Baldo, M. A.; O'Brien, D. F.; You, Y.; Shoustikow, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Nature 395, 151 (1998); O'Brien, D. F.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 74, 442 (1999)]. A multilayer device where the emitting layer of Alq3 is doped with PtOEP shows a strong emission at 650 nm that is attributed to the triplet excitons of PtOEP.
Cyclometalated iridium(III) is another class of materials used for high efficiency OLEDs, which is known to show intense phosphorescence. Baldo et al. reported the use of fac-tri(2-phenylpyridine)iridium(III) [Ir(ppy)3] as phosphorescent emitting material as a dopant in a 4,4′-N,N′-diarbazole-biphenyl (CBP) host to give high quantum efficiency OLED [Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 75, 4 (1999)]. In light of the rich photoluminescence properties of Ir(ppy)3, there has also been a growing interest in the incorporation of 2-phenylpyridine derivatives into iridium(III) center to prepare triplet emitters for OLED applications. Another example of triplet emitters is the sky-blue complex iridium(III) bis[2-(4′,6′-difluorophenyl)pyridinato-N,C2′]-picolinate [Ir(4,6-dFppy)2(pic)], which exhibits a very high photoluminescence quantum yield of about 60% in solution and nearly 100% in a solid film when doped into high triplet energy host [Rausch, A. F.; Thompson, M. E.; Yersin, H. Inorg. Chem. 48, 1928 (2009); Adachi, C.; Kwong, R. C.; Djurovich, P. I.; Adamovich, V.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 79, 2082 (2001); Kawamura, Y.; Goushi, K.; Brooks, J.; Brown, J. J.; Sasabe, H.; Adachi, C. Appl. Phys. Lett. 86, 071104 (2005)]. In spite of the extensive use of 2-phenylpyrdine and its derivatives in iridium(III) systems for the fabrication of OLEDs, the use of phosphors containing alternative metal centers with these ligands remains essentially unexplored and remains underdeveloped.
In addition to enhancing emission efficiencies, the ability to vary the emission color is desirable. Most approaches to color variance involve the use of different emission characteristics for color tuning. Examples that employ a single light-emitting material as dopant to generate more than one emission color are rare. Recent studies have shown that different emission colors from a single emissive dopant can be generated by using a phosphorescent material, by changing the bias direction or by changing the dopant concentration. Welter et al. reports the fabrication of a simple OLED consisting of semiconducting polymer PPV and phosphorescent ruthenium polypyridine dopant [Welter, S.; Krunner, K.; Hofstraat, J. W.; De Cola, D. Nature, 421, 54 (2003)]. At forward bias, red emission from the excited state of the phosphorescent ruthenium polypyridine dopant is observed, while the OLED emits a green emission at reverse bias where the lowest excited singlet state of PPV is populated. Adamovich et al. reports the use of a series of phosphorescent platinum(II) [2-(4,6-difluorophenyl)pyridinato-N,C2′]-β-diketones as single emissive dopants in OLED [Adamovich, V.; Brooks, J.; Tamayo, A.; Alexander, A. M.; Djurovich, P. R.; D'Andrade, B. W.; Adachi, C.; Forrest, S. R.; Thompson, M. E. New J. Chem. 26, 1171 (2002)]. Both blue emission from monomeric species and orange emission from the aggregates are observed in such OLED where the relative intensity of the orange emission increases as the doping level increases. As a result, the electroluminescence color can be tuned by changing the dopant concentration with equal intensities to the monomeric and aggregate bands. In both cases, the change of electroluminescence color in an OLED can be accomplished by varying the external stimulus or fabrication conditions while employing the same light-emitting material.
Even though there has been increasing interest in electrophosphorescent materials, particularly metal complexes with heavy metal centers, most efforts have been focused on the use of iridium(III), platinum(II) and ruthenium(II). Other metal centers have had very little attention. In contrast to isoelectronic platinum(II) compounds that are known to show rich luminescence properties, very few examples of luminescent gold(III) complexes have been reported, which probably stems from the presence of low-energy d-d ligand field (LF) states and the electrophilicity observed for gold(III) metal center. One way to enhance luminescence of gold(III) complexes is by introduction of strong σ-donating ligands, as first demonstrated by Yam et al. for stable gold(III) aryl compounds found to display interesting photoluminescence properties even at room temperature [Yam, V. W. W.; Choi, S. W. K.; Lai, T. F.; Lee, W. K. J. Chem. Soc., Dalton Trans. 1001 (1993)]. Another interesting donor ligand is the alkynyl group. Although the luminescence properties of gold(I) alkynyls have been extensively studied, the chemistry of gold(III) alkynyls has been essentially ignored, the exception being a brief report on the synthesis of an alkynylgold(III) compound of 6-benzyl-2,2′-bipyridine [Cinellu, M. A.; Minghetti, G.; Pinna, M. V.; Stoccoro, S.; Zucca, A.; Manassero, M. J. Chem. Soc. Dalton Trans. 2823 (1999)], but its luminescence behaviour has remained unexplored. Yam et al. discloses the synthesis of a series of bis-cyclometalated alkynylgold(III) compounds using various strong σ-donating alkynyl ligands with all compounds exhibiting rich luminescence behaviors at both room and low temperatures in various media [Yam, V. W.-W.; Wong, K. M.-C.; Hung, L.-L.; Zhu, N. Angew. Chem. Int. Ed. 44, 3107 (2005); Wong, K. M.-C.; Zhu, X.; Hung, L.-L.; Zhu, N.; Yam, V. W.-W.; Kwok, H. S. Chem. Commun. 2906 (2005); Wong, K. M.-C.; Hung, L.-L.; Lam, W. H.; Zhu, N.; Yam, V. W.-W. J. Am. Chem. Soc. 129, 4350 (2007)]. In addition, the utilization of these luminescent gold(III) compounds as phosphorescent dopant materials in OLEDs yields strong electroluminescence with high external quantum efficiencies of about 5.5%. These luminescence gold(III) compounds contain one tridentate ligand and at least one strong σ-donating group coordinated to a gold(III) metal center.